Laser light detectors have been available in the past for use in precisely determining the proper elevation on construction job sites. The standard method for using such detectors is to mount a rotating laser light source at a particular elevation on a construction job site, then mount the laser light detector on a piece of equipment (such as on the blade of a bulldozer) to let the operator of the equipment know precisely the elevation of the equipment while the it is in use. For example, the laser light detector could be mounted on a pole attached to the blade of a bulldozer, so the operator of the bulldozer could keep the blade at the correct position while grading the land to the precise elevation desired.
To be most effective, a laser light detector would have an easily viewable display that gives the elevation indication to a person who is sited a few feet from the detector. In addition, the detector would normally have some type of photodiode or other photo-detecting devices on all four corners of the detector's enclosure, so that it could detect laser light coming from any direction. Typical laser light detectors must operate within a one hundred millisecond cycle time, since most rotating laser light sources rotate at 600 rpm.
Typical laser sources used as rotating laser light sources operate in either the infrared or red light frequency spectrum. For example, infrared laser diodes operating at 780 nm are commonly used, as well as red light helium-neon gas lasers, operating at 633 nm. The laser light is typically collimated. Various rotating laser light sources are available having beam sizes from as small as one-quarter inch in diameter to as large as three-quarter inches in diameter.
Laser light detectors are typically available in two types of models: a "machine control receiver" and a "hand-held receiver". The machine control receiver is typically mounted on a piece of equipment, such as a bulldozer, and used in the manner discussed above. The hand-held receiver is typically a smaller device which can be carried by a typical construction worker to be used to detect the elevation of locations at a moment's notice.
A typical machine control receiver would use four light-sensitive arrays (one per side of the detector's enclosure), each having eight photodiodes arranged in a vertical linear manner. The eight photodiodes are typically arranged in pairs, thereby producing four channels of light input information. Each of the pairs of photodiodes are connected in parallel and drive a pulse amplifier, thereby converting the current pulse, generated as laser light sweeps across and is received by the photodiodes, into a voltage pulse. Since there are four channels of photodiodes, there would also be four channels of pulse amplifiers.
The voltage output of each pulse amplifier is directed into a comparator circuit, in which the peak value of the voltage pulse received from the pulse amplifier is compared to a voltage reference that is predetermined in the machine control receiver. If the received voltage pulse is of sufficient amplitude, the output of the comparator will change state and produce a logic pulse, e.g., by driving the input of a one-shot circuit (i.e., a monostable multivibrator circuit), which will have an output that changes state and holds that state for a time. Again, since there are four input channels, there would be four comparators and four latching circuits (such as one-shot circuits), one per input channel.
In the typical machine control receiver, the output of each of the four latching circuits is directed into a decoder circuit, which then determines the elevation that should be displayed by the machine control receiver. Once this decoding function has taken place, the latching circuits are all reset back to their initial zero output states. The decoder circuit uses a truth table to determine what the display should be showing. For example, if the photodiodes are arranged such that input channel number 1 is connected to the physically highest pair of photodiodes, then input channel 2 would be connected to the pair of photodiodes positioned just below, input channel 3 would be the next pair below that, and input channel 4 would be connected to the lowest pair of photodiodes. It is desirable that the positions of the photodiodes are arranged such that the diameter of the beam, as it strikes the photodiode array, will strike either one photodiode alone, or a pair of adjacent photodiodes. In other words, the photodiodes are arranged such that the light beam should not strike more than two photodiodes of the array at one time, however, some rotating light sources use wider beam widths which may cover more than two adjacent photodiodes.
Using the information gleaned from these four input channels, the decoder circuit of the machine control receiver can determine at least seven elevations of beam strike height, as follows: if input channel 4 alone receives a light pulse, then the "course high" indication is displayed; if both channels 3 and 4 are detecting light, then the "medium high" indication is displayed; if channel 3 alone receives a light pulse then the "fine high" indication is displayed; if both input channels 2 and 3 are receiving light (meaning that the light beam is striking the very center of the photodiode array), then the "on-grade" indication is displayed; if input channel 2 alone is receiving light, then the "fine low" indication is displayed; if both input channels 1 and 2 are receiving light, then the "medium low" indication is displayed, and finally; if channel 1 alone is receiving light then the "course low" indication is displayed. Naturally, if none of the input channels are receiving light, then the display would give no indication of elevation.
Using the above scheme of light detection, only seven discrete elevations of indication are possible. In addition, since the magnitude of the received light pulse, as its voltage equivalent is output from the pulse amplifier, is compared to a fixed voltage reference by the comparator, the machine control receiver of the prior art is greatly influenced by the energy level of the received light. Typical prior art rotating light sources can emit either infrared, or red light, as related hereinabove, and such infrared sources produce less power than the red light sources. This means that the machine control receiver will operate with different characteristics when used with an infrared source than when used with a red light source.
Another problem in using a machine control receiver of the prior art is the use of rotating light sources that have different spot sizes. Since the decoder depends upon the received light beams striking either one or two adjacent photodiodes at a particular time, the machine control receiver will exhibit different characteristics if the beam size changes from one rotating laser light source to another. In other words, it will be more difficult for two photodiodes to simultaneously receive a light pulse when the laser light source is outputting a very narrow beam width.
The other type of prior art laser light detector, the hand-held receiver, typically uses two photodiodes arranged one above the other along a vertical line, or uses a split single photodiode which produces two outputs. A typical laser light detector of this design is described in U.S. Pat. No. 4,676,634. The photodiode converts light energy into a current which is directed into the input of a pulse amplifier, which then converts the current into a voltage pulse. Since there are two current inputs, there are also two pulse amplifiers. The voltage output of each pulse amplifier is directed into a peak detector, which temporarily stores the peak magnitude of the voltage pulse that is output from each of the pulse amplifiers.
The output of the peak detector from the upper photodiode can be given the designation "A", and the output of the peak detector from the lower photodiode can be given the designation "B" Signals A and B are directed into a ratio comparator circuit which has four logic outputs. The first logic output will be activated if A&gt;K.sub.1 B. The second logic output would be activated if A&gt;K.sub.2 B. The third logic output would be activated if B&gt;K.sub.1 A, and the fourth logic output would be activated if B&gt;K.sub.2 A. Constants K.sub.1 and K.sub.2 are determined in advance by the hand-held receiver as fixed values. It can thus be seen that the logic output states are determined by the comparison of the intensity of signal A versus the intensity of signal B, to produce the desired logic output states. These four logic signals are directed into a decoder and latching circuit, which determines what elevation should be indicated on the visual display of the hand-held receiver.
The primary problem with such peak detecting units is the fact that their signal-to-noise ratio is poor. As system noise increases, the signal-to-noise ratio decreases. In fact, if the noise increases substantially to the point where it swamps out the signal, then the signal-to-noise ratio becomes nearly equal to 1.0 or less than 1.0, which is an extremely poor signal-to-noise ratio.
As related hereinabove, the laser light detectors presently available are either not reliable when the signal magnitude is diminished (by weather effects, for example) causing a poor signal-to-noise ratio (in the hand-held receiver, for example), or are greatly influenced by either the energy level received, or by a varying beam size of the light pulse striking the detector (in the case of the machine control receiver).